Power generated by renewable resources like wind and solar farms grew by more than 4 million megawatt hours in Texas in 2013, the largest increase in four years, according to a report from the state's grid operators released Friday.

The majority of generation gain came from wind farms, which continue to be built in large numbers in the Texas panhandle and along the Gulf Coast. Wind power represents the vast majority of the state's renewable portfolio - about 97 percent of the more than 38 million megawatt hours generated last year.

The report by the Electric Reliability council of Texas represents generators that participate in the Renewable Energy Credit Trading program, which was set up the Legislature in 1999 as a means to provide financial incentive for renewable development.

Solar power, while still a small percentage of the total generation, saw the largest gain of all the renewable sources. Generation from commercial solar farms and residential programs grew by a third to almost 180,000 megawatt hours.

And with a number of large scale farms under development, including a 150 megawatt farm announced Thursday by Recurrent Energy, that number is expected to grow exponentially in the years ahead.